While there's no gaurranty, one should feel free to use some common sense and judgement. For example, I wonder how many people who drop their helmets, immediately replace them.

Obviously, if there's visible damage, like a crush zone in the foam, or a crack or deep scratch in the shell, one would err to the side of caution. However, absent some evidence of damage, one should be able to assume that whatever happened falls within the area of general wear and tear that helmets are built to withstand.